The present invention relates to nondispersive infrared photometers of the antiphase-modulated single-beam type incorporating a detector comprised of two absorption chambers arranged one behind the other.
More specifically, the invention relates to a method of initially adjusting the photometer to render its measurements independent of variations in the composition of the carrier gas accompanying the gas whose concentration is actually to be determined.
Two basically different types of infrared photometers are known, the double-beam type and the single-beam type. These are respectively shown, in schematic form, in FIGS. 1a and 1b.
The double-beam analyzer of FIG. 1a includes two light sources LS, a reference cuvette RC, a measurement cuvette MC, and a detector arrangement comprised of two absorption chambers AC. The measurement cuvette MC contains a mixture of a gas whose concentration is to be measured accompanied by a carrier gas. Reference cuvette RC contains a reference gas. The two beams from the two sources LS are identically modulated (in-phase modulation) by a light chopper LC. The two beams pass through the respective cuvettes MC, RC, and energy in certain spectral portions of the light beam is selectively absorbed. The beams, after emerging from the cuvettes, enter the respective absorption chambers of the detector arrangement. The difference between the pressure pulses induced in the two gas-filled absorption chambers constitutes an indication of the difference between the gases in the two cuvettes, and is measured.
In contrast, the single-beam analyzer of FIG. 1b is provided with only a single light source LS, and a light chopper LC and detector arrangement designed differently than those of FIG. 1a. The light chopper LC modulates the light beams passing through the cuvettes MC and RC identically, but alternately (antiphase modulation). The detector arrangement is again comprised of two absorption chambers, but here arranged one behind the other. As with the double-beam analyzer of FIG. 1a, the difference between the pressure pulses induced in the two absorption chambers of the detector arrangement constitutes an indication of the difference between the gases in the measurement and reference cuvettes, and is measured.
A double-beam analyzer of the type shown in FIG. 1a is disclosed in detail, for example, in German Pat. No. 730,478, filed Mar. 8, 1938. A single-beam analyzer of the type shown in FIG. 1b is disclosed in detail, for example, in U.S. Pat. No. 3,162,761.
With the single-beam apparatus of FIG. 1b, the two absorption chambers are filled with gas, usually a mixture of the gas whose concentration is to be measured along with inert carrier gas. When the light beam passes through the measuring cuvette, there occurs therein a selective absorption of radiation at certain wavelengths corresponding to the infrared-active molecular resonances of the gas whose concentration is to be measured.
The beam emerging from the cuvette then passes through the two absorption chambers of the detector arrangement. In the front chamber, which is shorter than the back chamber, radiation energy is preferentially absorbed at frequencies corresponding to the middle portions of the absorption lines of the gas filling the absorption chamber; in the back absorption chamber, energy is mainly absorbed at frequencies corresponding to the flank portions of the absorption lines of the gas filling the absorption chamber. The absorbed electromagnetic energy is transformed within the absorption chambers into translational energy (heat), producing in the absorption chambers pressures indicative of the absorption energies. The difference between the absorption energies of the two absorption chambers is indicative of the concentration of the gas component of interest in the measurement cuvette.
If the null point of the apparatus is properly established, the output signal from the detector arrangement will be zero when the concentration of the gas component of interest in the measurement cuvette is zero. The null point compensation required to establish a proper null point conventionally involves proper choice of the dimensions of the measuring chamber and proper choice of the concentrations of gases therein.
With a single-beam antiphase-modulated apparatus as shown in FIG. 1b, if the null point has been properly established, the output signal of the detector arrangement will be constant if both the reference cuvette and the measurement cuvette are filled with the reference gas. The radiation passing through the two cuvettes alternately and then entering the common detector arrangement will be the same, so that in superimposition, a constant intensity is sensed by the detector arrangement.
Besides proper establishment of the null point of the gas concentration analyzer, another important operating characteristic is the selectivity of the analyzer. The selectivity of the detector is dependent upon the absorption spectrum of the gas filling the absorption chambers of the detector, and it may be increased by arranging a radiation filter in front of the detector. In recent years, use has been made of interference filters made up of multi-dielectric optically transparent layers of material the transmission of which is limited to predetermined wavelengths. However, the gas to be analyzed often includes, besides the component whose concentration is to be measured, other components whose absorption bands overlap those of the component whose concentration is to be measured. When this is the case, a cross-sensitivity effect may result. To counteract this, filter cuvettes may be interposed in the radiation path.
A further factor of importance to the accuracy of gas concentration measurements performed using such photometers is the so-called collision damping effect. This effect is attributable to the presence of gas components in the measurement cuvette which are not themselves infrared-active, and these components can affect the sharpness of the absorption lines associated with the gas component whose concentration is to be measured.
The collision broadening effect is very briefly explained as follows: The radiation incident in the measurement cuvette is absorbed in specific absorption bands of the gas component whose concentration is to be measured. Collision processes between the molecules which have absorbed radiant energy and other molecules result in a transformation of the absorbed energy into thermal energy within a very short time. The shape of the absorption curve of the gas component whose concentration is to be measured is ordinarily characterized by very fine-structured individual absorption lines, but these are appreciably influenced by such collisions with other molecules, even infrared-inactive molecules in the inert carrier gas in the measurement cuvette. During such a molecular colision, the natural absorption frequencies are temporarily changed by the mutual influence of electromagnetic fields, and the fine-structured absorption lines become temporarily broadened. This has a corresponding effect upon the accuracy and reliability of any measurements which may be performed.
There are additional effects tending to broaden the fine-structured absorption lines, including radiation broadening and Doppler broadening. However, I have found that collision broadening is usually the predominant line-broadening factor in the i.r. range.
As a result of the collision broadening effect, the measurement signal produced by the detector arrangement is not only dependent upon the partial pressure of the gas component whose concentration is to be measured; it is also dependent upon the composition of the carrier gas in the measurement cuvette, even if the carrier gas is entirely inert and exhibits no infrared activity of its own.
The collision broadening effect, because it is attributable to collisions with carrier gas molecules, becomes a very troublesome factor when the gas in the measuring cuvette includes carrier gas components of widely and quickly fluctuating composition. This is the case, for example, when analyzing blast furnace gas, or when analyzing human breath during anesthesia.
In such situations, it may happen that the concentration of the gas component of interest does not change during a certain period of time, but that fluctuations in the composition of the accompanying carrier gas will produce corresponding fluctuations in the output signal of the detector arrangement. For that reason, the reading provided by a photometer in such a situation cannot be considered accurate except within a range corresponding to the possible effect of carrier-gas-composition fluctuations. Therefore, it may be difficult or impossible to reliably detect fluctuations in the concentration of the gas component of interest, if the fluctuations are smaller than would correspond to the possible effect of the fluctuations in the composition of the accompanying carrier gas.
The collision broadening effect also produces difficulties in calibrating the photometer. Conventionally, calibration is performed in the following way. A plurality of calibrating gas samples are obtained. Each calibrating gas sample includes as one component the gas whose concentration is to be measured, and it also includes inert carrier gas of often different compositions from those expected to be encountered during the post-calibration measuring operations. The output signal produced by the two-absorption-chamber detector is noted for each of the different calibrating gas samples, so that during actual measurement there will be available a way of correlating the output signal with known concentrations of the gas whose concentration is to be measured.
This calibrating procedure is complicated by the collision broadening effect, i.e., by the effect of fluctuations in the composition of the infrared-inactive carrier gas in the calibrating gas samples, and in the gas in the measurement cuvette during actual measurements. If two different photometers have been calibrated using calibrating gas samples of non-identical carrier gas composition, the correlation between the detector output signal and the gas concentration reading of one photometer will be different from that of the other photometer. As a result, a series of measurements begun on one photometer cannot be readily continued on the other photometer. Likewise, comparison of the results achieved using one photometer cannot be readily made using the other photometer. This makes it necessary to recalibrate one of the photometers, or else to keep available correlation tables, or the like, to be able to correlate the readings of one instrument with that of the other. Actually, although the invention relates to single-beam analyzers (FIG. 1b), this problem is present with double-beam analyzers (FIG. 1a), as well.
FIG. 2 depicts the effect of fluctuations in carrier gas composition upon the actual gas-concentration measurement for a double-beam analyzer (curve a) and for a single-beam analyzer (curve b). The gas component whose concentration is to be measured (in ppm) is CO.sub.2. The carrier gas with which the CO.sub.2 in the measurement cuvette is mixed during actual testing is itself a mixture of O.sub.2 and N.sub.2. However, the relative proportions of O.sub.2 and N.sub.2 in the carrier gas are assumed to fluctuate greatly during testing. In FIG. 2, CO.sub.2 concentration departure from the valve obtained for zero O.sub.2 concentration is plotted along the vertical axis, and the proportion of O.sub.2 in the O.sub.2 /N.sub.2 carrier-gas mixture is plotted along the horizontal axis. In this case the actual concentration of CO.sub.2 in the CO.sub.2 /O.sub.2 /N.sub.2 mixture was 320 ppm.
It will be noted that the detector output signal correctly indicates departure ppm CO.sub.2 only when the carrier gas consists entirely of N.sub.2 (the O.sub.2 fraction is zero); this is true both for the double-beam analyzer (curve a) and the single-beam analyzer (curve b). As the fraction of O.sub.2 in the O.sub.2 /N.sub.2 carrier-gas mixture increases from 0 to 1.0, the detector output signal changes appreciably, even though the actual CO.sub.2 concentration in the measurement cuvette has not left 320 ppm. For the double-beam analyzer (curve a), with its two absorption chambers arranged side by side, the detector output signal decreases as the O.sub.2 content in the carrier gas rises. For the single-beam analyzer (curve b), with its two absorption chambers arranged one behind the other, the detector output signal increases as the O.sub.2 content in the carrier gas rises. Clearly, fluctuations in the composition of the carrier gas have a very appreciable effect upon the ultimate measurement, in such a situation.